1. Field of the Invention
The present invention relates to a semiconductor device which is incorporated in an active matrix liquid crystal display element where polycrystalline Si thin film transistors (each referred to as "TFT" hereinafter) are used as switching elements for pixels.
2. Description of the Prior Art
FIG. 2 shows a sectional structure of a TFT used for a conventional active matrix liquid crystal display device as disclosed in Japanese Patent Laying-Open Gazette No. 2-72392 (1990). Such a structure is comprised of an insulating substrate 1 of transparent glass or the like, a polycrystalline Si film 2, a doped Si region 3 with doped impurity by means of ion implantation to make metal and Si into contact, a gate insulating film 4 formed on the polycrystalline Si film 2, a gate electrode 5 formed on the gate insulating film 4, a protecting film 6 formed of insulating thin film for covering at least part of a TFT 15 consisting of the polycrystalline Si film 2, the gate insulating film 4 and the gate electrode 5. Provided also therein are a contact hole 9 defined in the protecting film 6 on the Si region 3 which is doped with the impurity, a drain line 10, a source line 11, and a pixel electrode 12 connected to the drain line 10. Further provided therein is a storage capacitor 16 consisting of a line 7, an insulating film 8 and the pixel electrode 12. As to the active matrix liquid crystal display element, its equivalent circuit is shown in FIG. 3, where the TFT 15 is positioned at a crossing of the source line 11 and a gate line 5a. The TFT 15 has its drain connected to the pixel electrode 12 and the storage capacitor 16.
Then, an operation of the device will be described. Voltage applied to the gate electrode 5 is varied to cause electric field across an inner portion of the polycrystalline Si film 2 underlying the gate insulating film 4 to vary, and consequently, current flowing through the contact hole 9 and the doped Si region 3 between the source line 11 and the drain line 10 is controlled. Thus, a transistor operation is implemented. Voltage is applied to the gate electrode 5 and the source line 11 to turn the TFT 15 on for its transistor operation, and the ON-state of the TFT 15 serving as a switch allows voltage to be applied to a liquid crystal unit on the pixel electrode 12 so that a state of liquid crystal molecules is changed to control an amount of light transmission. Liquid crystal is provided over the entire region of the structure shown in FIG. 2, but omitted in the drawings for simplification. The protecting film 6 is a film for protecting the TFT 15 from external contaminants and the like. The protecting film 6 is used as a layer insulating film at the crossing of the source line 11 and the gate line 5a. The storage capacitor 16 shown in FIG. 3 is comprised of the line 7, the insulating film 8 and the pixel electrode 12. The storage capacitor 16 functions to increase a load capacity observed on the side of the TFT 15, to reduce a DC voltage component applied to the liquid crystal and to lighten disadvantage about display characteristics, such as a residual image and the like. The insulating film 8 for the storage capacitor is used as a layer insulating film at the crossing of the source line 11 and the gate line 5a. The drain line 10 is connected to the pixel electrode 12. The pixel electrode 12 is formed of a transparent conductive film of ITO or the like, and it functions to apply voltage to the liquid crystal and to transmit visible light.
A conventional semiconductor device is configured as mentioned above. In such a semiconductor device including a plurality of such TFTs 15, one of the TFTs 15 has its pixel electrode 12 positioned in the same plane as the source line 11 which is connected to the adjacent TFT on the right side, as shown in FIG. 3, and hence, when some failure occurs in photolithography and etching process for patterning, for example, they are short-circuited and accordingly a malpractice of display may be caused. As to the contact hole 9, since the depth thereof is so large as equal to the sum of a thickness of the protecting film 6 added to a thickness of the insulating film 8 for the storage capacitor, breaking of the drain line 10 and the source line 11 often arises at an edge of the contact hole 9 when the source electrode and the drain electrode are formed. In the case where Al is used for the drain and source lines 10, 11 for example, the source and drain lines 11, 10 are formed, and thereafter, annealing process at 400.degree. C. or over is often performed to improve an Ohmic characteristic with the doped Si region 3. However, once the annealing process is performed at a temperature of about 400.degree. C. or over, hydrogen is generally released, which is contained in the Si film to conduct a hydrogen treatment with terminating dangling bonds existing in a grain boundary thereof for improvement of TFT characteristics. Then, such a hydrogen treatment must be performed after the formation of the source and drain lines 11, 10. In the case where the storage capacitor 16 is formed of SiN or the like which is high in dielectric constant and which facilitates an acquisition of a relatively high storage capacity value, since an SiN film remains on the TFT 15 and a diffusion coefficient of hydrogen in SiN is small, a hydrogen treatment cannot be performed after the formation of the insulating film 8 for the storage capacitor, and there arises the problem that characteristics of the resultant TFT are not so good as that of the TFT to which such a hydrogen treatment has been performed.